This invention relates to a power window control for a motor vehicle and particularly to a control for a power window drive including a flexible drive element between the drive motor and window. An example of a flexible drive element is the drive tape used in some power window mechanisms used on vehicles sold by the assignee of this patent. The flexible drive element is placed in tension as the motor moves the window up or down. When the window reaches its full up or down position it stops and stalls the motor. The motor is then deactivated; but, if no means are provided for releasing the tension of the flexible drive element, it will remain in tension essentially all the time. It would be advantageous in increasing the reliability and useful life of the flexible drive element to provide tension relieving means in the motor control.
Tension relieving means for drive elements are known in the prior art. For example, the U.S. patent to Giles III, U.S. Pat. No. 4,246,520, issued Jan. 20, 1981 shows a vehicle power seat control in which the drive motor is automatically reversed for a predetermined time when operation is stopped in order to relieve pressure on the gear train and thus prevent a locked rotor condition. In addition, the U.S. patent to Comeau, U.S. Pat. No. 4,471,275, issued Sept. 11, 1984, shows a drapery closure control in which the drive motor is reversed for a predetermined time before stopping in order to relieve tension on the draw cords. In each of these controls the time duration of motor reversal is a constant predetermined duration, set by an RC time delay circuit. However, the constant predetermined duration of motor reversal, when applied to a vehicle power window drive, might produce a variable degree of reverse movement under different environmental and electrical operating conditions. It is desired that such a control, particularly in the window closing operation, always produce just enough reverse movement to relieve the tension in the drive elements but not enough to move the window itself away from its tightly closed position.